The invention relates to an optical arrangement, in particular a projection system, an illumination system or a beam shaping system for EUV lithography, to an EUV lithography device comprising such an optical arrangement, to methods for operating such an optical arrangement and an EUV lithography device, respectively, as well as to methods for cleaning and for providing a reflective optical element, respectively.
Reflective optical elements for the extreme ultraviolet (EUV) range and the soft X-ray wavelength range (e.g. wavelengths between approximately 5 nm and 20 nm) such as photomasks or multilayer mirrors are in particular used in EUV lithography devices for producing semiconductor components. Since EUV lithography devices as a rule comprise several reflective optical elements, the reflectivity of the latter has to be as high as possible in order to ensure sufficiently high overall reflectivity. The reflectivity and thus the service life of the reflective optical elements can be reduced as a result of contamination of the optically active reflective surface of the reflective optical elements, which contamination arises as a result of short-wave radiation together with residual gases in the operating atmosphere. Since normally in an EUV lithography device several reflective optical elements are arranged one behind the other, even minor contaminations on each reflective optical element have a major effect on the overall reflectivity. It should be pointed out that hereinafter the term “light” refers not only to radiation at wavelengths in the visible spectrum, but in particular also to radiation in the EUV- or in the soft X-ray wavelength range.
Apart from the intensity of the EUV radiation, the partial pressure of the hydrocarbons in the environment close to the optically active surface plays a significant role in the growth of contamination. If a hydrocarbon molecule is adsorbed on the optical surface of an element, that is reflective to EUV radiation, said hydrocarbon molecule either directly condensates and/or reacts with the photons of the exposure radiation or the radiation-induced electrons (photo electrons) and forms, for example, an atomic carbon layer, wherein either case results in a loss of reflection on the reflective optical element.
Depending on the composition of the residual gas atmosphere, as described above, the contamination can contain carbon or hydrocarbon, or it can be somewhat oxidative, or may contain volatile metal hydrides which may lead to metal depositions on the optical surfaces. In order to counteract the contamination, above all, attempts are made to provide the reflective optical elements with protective layers that are more inert in relation to the respective residual gases than is the surface of the optically active area of the reflective optical elements. Furthermore, different cleaning methods are investigated in more detail, by means of which cleaning methods contamination is to be removed from the reflective optical elements without the optical characteristics of said optical elements being significantly impeded. For example, the approach is pursued wherein contamination comprising carbon is removed in that atomic hydrogen is introduced into a cleaning chamber, which atomic hydrogen reacts, in particular with the contamination that contains carbon on the surface of the reflective element that is located in the cleaning chamber, to form volatile compounds. To this effect the atomic hydrogen is obtained e.g. by heating molecular hydrogen to approximately 2400° C. by means of a heated filament.
However, due to its high reactivity, atomic hydrogen is difficult to handle. Producing it by means of a heated filament is also associated with a danger of introducing additional contamination, which originate, for example, from the heated filament itself.
From WO 2002/054115 A2 a device has become known in which an additional protective layer, e.g. of ruthenium, rhodium, palladium, iridium, platinum, and/or gold is applied to the reflective optical element. The protective layer can have a catalytic effect and can stimulate the reaction of gaseous substances e.g. molecular hydrogen with EUV radiation, which substances are supplied to the optical surfaces, so that carbon contamination deposited on the protective layer can be removed. It is proposed that to this effect the gaseous substance be supplied to the optical surface at pressures of between 10−8 and 10−4 torr.
Furthermore, at present, in EUV lithography devices, reflective optical elements are used at an operating temperature of approximately 60° C. or less, because at higher temperatures the diffusion between the individual layers of the reflective multilayer systems greatly increases at the optical surfaces of said multilayer systems, which leads to a reduction in the reflectivity. For this reason the components which are present in the vacuum housings of EUV lithography devices, which components outgas contaminating substances such as hydrocarbons, cannot be completely baked. As a result of this, in those instances the partial pressure of the hydrocarbons can only be reduced at considerable expenditure, which as a rule is not tolerable, e.g. with longer pumping periods, to values below 10−9 mbar so that there is a high probability of contamination, in particular non-volatile hydrocarbons, adhering to the optical surfaces, and/or in the immediate environment of the optical surfaces of the reflective optical elements, or that said contamination is adsorbed there.
From WO 2004/104707 A2 a method and a device for cleaning at least one reflective optical element has become known, which optical element is arranged in a vacuum chamber in the beam path of a radiation source for generating radiation in the soft X-ray wavelength range or in the EUV wavelength range. The reflective optical element is contaminated, at least in part, as a result of an inorganic substance introduced through the radiation source. Depending on the prevailing reaction conditions, at least one reaction partner, which is transparent in relation to the radiation, is admitted by way of a supply device, which reaction partner chemically reacts with the contaminating deposits for the purpose of removing them from the reflective optical element. The reaction partner can, for example, be molecular hydrogen, and for the purpose of desorption of the contamination the reflective optical element can be heated.